National Transportation Safety Board investigators say under Bay Area Rapid Transit rules, the two track workers who were killed in an accident Saturday were responsible for their own safety.

James Southworth, the NTSB’s lead investigator, said at a briefing Tuesday that the workers had requested approval from the control center to go onto the tracks. The type of approval they were given required them to make sure they remained out of danger as they worked. Under BART’s rules, one of the two workers was to be a lookout positioned outside the right of way to warn the other of an approaching train.

Southworth says the investigation showed the train’s wheels showed signs of flattening from when the operator used the emergency brake in an attempt to avoid the fatal accident.